Field of the Invention
This invention relates to an information recording magnetic medium in the form of a disk for use with a Winchester type fixed disk device, whereby writing and reading of information is attained by a floating head.
Description of the Background
In a magnetic recording medium such as a disk, a magnetic layer of, for example, an iron oxide or a Co alloy is formed on a non-magnetic base plate by a sputtering method or a plating method. At the start or stop of the rotation of the disk, a floating head of a fixed disk device is mechanically brought into contact with the magnetic surface of a magnetic recording medium. Thus, the surface of the magnetic layer is covered by a protective layer so that the magnetic layer is not injured or a lubricating layer is formed on the magnetic layer for attaining good floating of the floating head. That is, in a fixed disk device, the head is in contact with a recording medium when the device is stopped. When the device is started, and during the change of the rotation of the disk in the range of 0 to 3,600 r.p.m., the head switches from a contact running mode to a float running mode.
On the other hand, when the device is stopped and during the change of the rotation of the disk in the range of 3,600 to 0 r.p.m., the head switches from the float running mode to the contact running mode. The head is then brought into contact with the disk at 0 r.p.m. Such a process is generally called "contact start stop" (CSS) and for CSS resistant characteristics, more than 10,000 rotations, preferably more than 20,000 rotations are required. This is considered to be an important characteristic of the reliability of a magnetic recording medium and hence of a fixed disk device.
For improving the CSS resistant characteristics, it is necessary to reduce the friction coefficient between the head and the magnetic recording medium as low as possible. This, in turn, reduces the occurrence of abrasion of the head or the magnetic recording medium. It is also necessary to protect the magnetic layer from mechanical damage during contact with the floating head by covering the magnetic layer with a layer having a mechanical character capable of avoiding injury to the ferrite head. Furthermore, when a magnetic layer is formed by a thin layer of a metal such as a Co-base alloy, the magnetic layer has a strong corrosive property and thus is liable to form a non-magnetic oxide. Accordingly, a protective layer is usually formed on the magnetic layer for improving the corrosion resistance thereof.
It is known to utilize a protective layer and a lubricating layer formed by forming a SiO.sub.2 layer by a spin coating method or a sputtering method and coating thereon a prefluoropolyether liquid lubricant. It is also known to utilize a protective layer and a lubricating layer formed by forming a SiO.sub.2 layer as described above and covering the layer thereof with a prefluoropolyether solid lubricant. Also known is the utilization of a protective and lubricating layer formed by forming an amorphous carbon layer on the magnetic layer by a sputtering method.
However, the protective and lubricating layer having the above-described construction have the following disadvantages.
The construction of a coating with a liquid lubricant on the SiO.sub.2 layer has the disadvantage that the ratio of the viscosity of the lubricant and the coating amount thereof to attain good lubrication is delicate and hence it is not easy to establish stable coating conditions. In general, if the coating amount of a liquid lubricant is increased, the friction coefficient between a head and the magnetic recording medium is also increased. If the viscosity of a liquid lubricant is low, the coated layer shows good friction characteristics even when the coated amount thereof is increased. Hence, in the case where a liquid lubricant is used, even if the optimum conditions for the viscosity of the lubricant and the coating amount thereof has been attained the optimum conditions differ according to the condition of the surface of the base plate and the environmental conditions (in particular, humidity).
In addition, since such liquid lubricants are generally coated by a spin coating method or a dip coating method, the coating is liable to become uneven. Furthermore, when a suitable amount of a liquid lubricant is coated under conditions of high humidity and a head is placed on the magnetic recording medium in a contact stop state for a long period of time, the static friction force is greatly increased. This is considered to be caused by the existence of the lubricant between the sliding surface of the head having high surface smoothness and the surface of the magnetic recording medium. This is a phenomenon similar to the so-called linking phenomenon whereby block gages closely adhere to each other when lubrication between them occurs. The static friction force is called "adsorptive force" and when the adsorptive force becomes greater than a certain value (e.g., 6 gf), a load is applied to the head spring at the start which causes buckling. This results in injury to the surface of the magnetic recording medium by the slider end of the head. Also, if this occurrence is increased, it causes a fatal damage called "head crash" to both, the head and the magnetic recording medium.
As described above, the use of a liquid lubricant has the disadvantages that the management of the coating conditions is not easy, that such lubricant shows large dispersion for the quality thereof, and that the adsorptive force is liable to increase.
Coating a solid lubricant on a SiO.sub.2 layer has the disadvantages that adhesion between the SiO.sub.2 layer and the solid lubricant layer is poor and the lubricant layer lacks reliability for long periods of time. Also, for attaining good adhesion between both the layers, a very complicated step is required, for example, as follows. Si(OH).sub.4 is coated on a magnetic layer and baked to form a SiO.sub.2 layer on the surface of the magnetic layer. This layer becomes porous by elimination of water at baking, which reduces the surface preciseness (smoothness). The surface quality of such a porous SiO.sub.2 layer is improved by applying, for example, tape polishing. Then, after coating thereon a solid lubricant, buffing is applied to fix the solid lubricant into the pore portions. Then, the solid lubricant is welded onto the SiO.sub.2 layer by baking. The protective and lubricating layers thus formed may show excellent lubricating property but the formation step thereof is complicated as described above, which is unsuitable for mass production.
An amorphous carbon layer can be formed by sputtering. The thus formed layer can be utilized as the protective and lubricating layer of a magnetic layer. In particular when a thin metal layer is used as the magnetic layer, the amorphous carbon layer has the disadvantage that it does not protect the magnetic layer from corrosion.